A planar type transformer (hereinafter referred to as a planar transformer) may be reduced in size compared with the winding transformers in related art, and so in recent years, the planar transformer has found increasing use in switching power supply circuits of various information apparatuses.
The planar transformer has a structure in which primary and secondary winding patterns (hereinafter referred to as a primary main winding pattern and a secondary main winding pattern) are stacked with an insulating layer interposed between the primary and secondary main winding patterns that serve as the main windings for performing power transmission. A planar transformer provided with an auxiliary winding pattern has been presented, the auxiliary winding pattern being used for power transmission between the auxiliary winding pattern and the primary or secondary main winding pattern. The auxiliary winding pattern is used, for example, for power supply to a control circuit that controls a switching element that switches to determine whether or not a switching current is supplied to the primary main winding pattern.
In order to reduce the resistance loss of a planar transformer, it is desirable that the line width (pattern width) of each main winding pattern be increased as much as possible and the main winding pattern is disposed in multiple layers, and consequently the number of layers tends to increase. Further addition of an auxiliary winding pattern is accompanied by additional formation of an insulating layer or the like (when an odd number of winding pattern layers is provided, a dummy layer (described later) for avoiding curling of a multilayer substrate is also added), thereby increasing the height of the multilayer substrate, and thus the planar transformer is increased in size.
The following are reference documents.
[Document 1] Japanese Laid-open Patent Publication No. 2002-270437 and
[Document 2] Japanese Laid-open Patent Publication No. 2010-93174.